This invention relates to an air/fuel ratio control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to such a system which is adapted to automatically close exhaust gas recirculation valve (EGR valve) which returns exhaust gases from the exhaust system of the engine to the intake system, to cause interruption of the exhaust gas recirculation at low engine load or at low engine temperature during air/fuel ratio control operation, to thereby prevent incomplete combustion within engine cylinders.
An air/fuel ratio control system has been proposed by the assignee of the present application, which is adapted to perform feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine having an intake system and an exhaust system, and which comprises means for detecting the concentration of an exhaust gas ingredient emitted from the engine, fuel quantity adjusting means for producing the mixture being supplied to the engine, and an electrical circuit operatively connecting the concentration detecting means with the fuel quantity adjusting means in a manner effecting feedback control operation to control the air/fuel ratio of the mixture to a predetermined value in response to an output signal produced by the concentration detecting means.
An exhaust gas recirculation system (EGR system) is already known, which is provided at the exhaust passage of the engine to introduce exhaust gases as inert gases into the intake passage of the engine for mixture with the suction mixture so as to cause a drop in the maximum combustion temperature within the engine cylinders and accordingly a reduction in the amount of NOx to be present in the exhaust gases. When this exhaust gas recirculation is effected by means of such exhaust gas recirculation system, there can occur incomplete combustion within engine cylinders during low load operation of the engine such as idle and deceleration and during low temperature operation of the engine. Therefore, a conventional system of this kind is adapted to have its exhaust gas recirculation operation interrupted during such engine operations. To determine such engine operating conditions for interruption of the exhaust gas recirculation, the conventional system is provided with a specially made pressure sensor having an atmospheric pressure-compensating bellows for detecting the low load condition of the engine, as well as a specially made thermosensor for detecting the low load condition of the engine.